Tie mip map cache purging to Images, not Bitmaps/PixelRefs

We were potentially using different keys for lookup vs. add, because we
were adding with a key based on the pixelRef, which may not have the
same uniqueID as the originating image.

Bug: skia:
Change-Id: Ib4d3d5ead9f5a574cf6d1920080bc9c4ae66c1d0
Reviewed-on: https://skia-review.googlesource.com/c/161625
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed